Apparatus and method for handling BTAC branches that wrap across instruction cache lines

ABSTRACT

A branch control apparatus in a microprocessor. The apparatus includes a branch target address cache (BTAC) that caches indications of whether a branch instruction wraps across two cache lines. When an instruction cache fetch address of a first cache line containing the first part of the branch instruction hits in the BTAC, the BTAC outputs a target address of the branch instruction and indicates the wrap condition. The target address is stored in a register. The next sequential fetch address selects a second cache line containing the second part of the branch instruction. After the two cache lines containing the branch instruction are fetched, the target address from the register is provided to the instruction cache in order to fetch a third cache line containing a target instruction of the branch. The three cache lines are stored in order in an instruction buffer for decoding.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/898,583 filed Jul. 3, 2001.

FIELD OF THE INVENTION

This invention relates in general to the field of branch target addresscaching in pipelined microprocessors, and more particularly to branchinstructions that wrap across instruction cache lines.

BACKGROUND OF THE INVENTION

Pipelined microprocessors include multiple pipeline stages, each stageperforming a different function necessary in the execution of programinstructions. Typical pipeline stage functions are instruction fetch,instruction decode, instruction execution, memory access, and resultwrite-back.

The instruction fetch stage fetches the next instruction in thecurrently executing program. The next instruction is typically theinstruction with the next sequential memory address. However, in thecase of a taken branch instruction, the next instruction is theinstruction at the memory address specified by the branch instruction,commonly referred to as the branch target address. The instruction fetchstage fetches instructions from an instruction cache. If theinstructions are not present in the instruction cache, they are fetchedinto the instruction cache from another memory higher up in the memoryhierarchy of the machine, such as from a higher-level cache or fromsystem memory. The fetched instructions are provided to the instructiondecode stage.

The instruction decode stage includes instruction decode logic thatdecodes the instruction bytes received from the instruction fetch stage.In the case of a processor that supports variable length instructions,such as an x86 architecture processor, one function of the instructiondecode stage is to format a stream of instruction bytes into separateinstructions. Formatting a stream of instructions includes determiningthe length of each instruction. That is, instruction format logicreceives a stream of undifferentiated instruction bytes from theinstruction fetch stage and formats, or parses, the stream ofinstruction bytes into individual groups of bytes. Each group of bytesis an instruction, and the instructions make up the program beingexecuted by the processor. The instruction decode stage may also includetranslating macro-instructions, such as x86 instructions, intomicro-instructions that are executable by the remainder of the pipeline.

The execution stage includes execution logic that executes the formattedand decoded instructions received from the instruction decode stage. Theexecution logic operates on data retrieved from a register set of theprocessor and/or from memory. The write-back stage stores the resultsproduced by the execution logic into the processor register set.

An important aspect of pipelined processor performance is keeping eachstage of the processor busy performing the function it was designed toperform. In particular, if the instruction fetch stage does not provideinstruction bytes when the instruction decode stage is ready to decodethe next instruction, then processor performance will suffer. In orderto prevent starvation of the instruction decode stage, an instructionbuffer is commonly placed between the instruction cache and instructionformat logic. The instruction fetch stage attempts to keep severalinstructions worth of instruction bytes in the instruction buffer sothat the instruction decode stage will have instruction bytes to decode,rather than starving.

Typically, an instruction cache provides a cache line of instructionbytes, typically 16 or 32 bytes, at a time. The instruction fetch stagefetches one or more cache lines of instruction bytes from theinstruction cache and stores the cache lines into the instructionbuffer. When the instruction decode stage is ready to decode aninstruction, it accesses the instruction bytes in the instructionbuffer, rather than having to wait on the instruction cache.

The instruction cache provides a cache line of instruction bytesselected by a fetch address supplied to the instruction cache by theinstruction fetch stage. During normal program operation, the fetchaddress is simply incremented by the size of a cache line since it isanticipated that program instructions are executed sequentially. Theincremented fetch address is referred to as the next sequential fetchaddress. However, if a branch instruction is decoded by the instructiondecode logic and the branch instruction is taken (or predicted taken),then the fetch address is updated to the target address of the branchinstruction (modulo the cache line size), rather than being updated tothe next sequential fetch address.

However, by the time the fetch address is updated to the branch targetaddress, the instruction buffer has likely been populated withinstruction bytes of the next sequential instructions after the branchinstruction. Because a branch has occurred, the instructions after thebranch instruction must not be decoded and executed. That is, properprogram execution requires the instructions at the branch target addressto be executed, not the next sequential instructions after the branchinstruction. The instruction bytes in the instruction buffer wereerroneously pre-fetched in anticipation of the more typical case ofsequential instruction flow in the program. To remedy this error, theprocessor must flush all instruction bytes behind the branchinstruction, which includes the instruction bytes in the instructionbuffer.

Flushing the instruction buffer upon a taken branch instruction iscostly since now the instruction decode stage will be starved until theinstruction buffer is re-populated from the instruction cache. Onesolution to this problem is to branch prior to decoding the branchinstruction. This may be accomplished by employing a branch targetaddress cache (BTAC) that caches fetch addresses of instruction cachelines containing previously executed branch instructions and theirassociated target addresses.

The instruction cache fetch address is applied to the BTAC essentiallyin parallel with the application of the fetch address to the instructioncache. In the case of an instruction cache fetch address of a cache linecontaining a branch instruction, the cache line is provided to theinstruction buffer. In addition, if the fetch address hits in the BTAC,the BTAC provides an associated branch target address. If the branchinstruction hitting in the BTAC is predicted taken, the instructioncache fetch address is updated to the target address provided by theBTAC. Consequently, the cache line containing the target instructions,i.e., the instructions at the target address, will be stored in theinstruction buffer behind the cache line containing the branchinstruction.

However, the situation is complicated by the fact that in processorsthat execute variable length instructions, the branch instruction maywrap across two cache lines. That is, the first part of the branchinstruction bytes may be contained in a first cache line, and the secondpart of the branch instruction bytes may be contained in the next cacheline. Therefore, the next sequential fetch address must be applied tothe instruction cache rather than the target address in order to obtainthe cache line with the second part of the branch instruction. Then thetarget address must somehow be applied to the instruction cache toobtain the target instructions.

Therefore, what is needed is a branch control apparatus that providesproper program operation in the case of wrapping BTAC branches.

SUMMARY

The present invention provides a branch control apparatus in a pipelinedprocessor that provides proper program operation in the case of wrappingBTAC branches. Accordingly, in attainment of the aforementioned object,it is a feature of the present invention to provide a branch controlapparatus in a microprocessor having an instruction cache, coupled to anaddress bus, for providing cache lines to an instruction buffer. Theapparatus includes a target address of a branch instruction. A branchtarget address cache (BTAC) provides the target address. The apparatusalso includes a wrap signal, provided by the BTAC, which indicateswhether the branch instruction wraps across first and second cachelines. The apparatus also includes an address register, coupled to theBTAC, that stores the target address. If the wrap signal indicates thebranch instruction wraps across the first and second cache lines, theaddress register provides the target address on the address bus to theinstruction cache to select a third cache line. The third cache linecontains a target instruction of the branch instruction.

In another aspect, it is a feature of the present invention to provide apipelined microprocessor. The microprocessor includes an instructioncache, coupled to an address bus that receives a first fetch address forselecting a first cache line. The microprocessor also includes a branchtarget address cache (BTAC), coupled to the address bus, which providesa wrap indicator for indicating whether a branch instruction wrapsbeyond the first cache line. The microprocessor also includes an addressregister, coupled to the BTAC, that stores a target address of thebranch instruction. The target address is provided by the BTAC. Themicroprocessor also includes a multiplexer, coupled to the BTAC, whichselects a second fetch address for provision on the address bus if thewrap indicator is true. The second fetch address selects a second cacheline containing a portion of the branch instruction wrapping beyond thefirst cache line. The multiplexer selects the target address from theaddress register for provision on the address bus after selecting thesecond fetch address for provision on the address bus.

In another aspect, it is a feature of the present invention to provide abranch control apparatus in a microprocessor. The branch controlapparatus includes a branch target address cache (BTAC) that cachesindications of whether previously executed branch instructions wrapacross two cache lines. The branch control apparatus also includes aregister, coupled to the BTAC, that receives from the BTAC a targetaddress of one of the previously executed instructions. The branchcontrol apparatus also includes control logic, coupled to the BTAC, thatreceives one of the indications. If the one of the indications indicatesthe one of the previously executed branch instructions wraps across twocache lines, the control logic causes the microprocessor to branch tothe target address, after causing the two cache lines containing the oneof the previously executed branch instructions to be fetched.

In another aspect, it is a feature of the present invention to provide amicroprocessor branch control apparatus. The branch control apparatusincludes an incrementer, coupled to an instruction cache address bus,that provides a first fetch address on the address bus. The first fetchaddress selects a first cache line containing a first portion of abranch instruction. The branch control apparatus also includes a branchtarget address cache (BTAC), coupled to the address bus, which providesa target address of the branch instruction in response to the firstfetch address. The branch control apparatus also includes an addressregister, coupled to the BTAC, that stores the target address if theBTAC indicates the branch instruction wraps beyond the first cache line.The incrementer provides a second fetch address on the address bus. Thesecond fetch address selects a second cache line containing a secondportion of the branch instruction. The address register provides thetarget address on the address bus. The target address selects a thirdcache line containing a target instruction of the branch instruction.

In another aspect, it is a feature of the present invention to provide amethod for performing branches in a microprocessor with an instructioncache. The method includes applying a first fetch address to theinstruction cache for selecting a first cache line containing at least aportion of a branch instruction, providing a target address of thebranch instruction in response to the first fetch address, anddetermining whether the branch instruction wraps beyond the first cacheline. The method also includes storing the target address in a registerif the branch instruction wraps beyond the first cache line, applying asecond fetch address to the instruction cache, if the branch instructionwraps beyond the first cache line, for selecting a second cache linecontaining a remainder of the branch instruction, and providing thetarget address from the register to the instruction cache for selectinga third cache line containing a target instruction of the branchinstruction.

An advantage of the present invention is that it potentially improvesbranch performance in a pipelined microprocessor that uses a BTAC byenabling the processor to take a BTAC branch even if the branch wrapsacross multiple cache lines. The invention enables wrapped branchingeven in processors that do not have stalling circuitry in the pre-decodestages of the processor, thereby avoiding the branch penalty associatedwith mispredicting the branch as not taken and subsequently correctingfor the misprediction. The avoidance of the branch penalty isparticularly advantageous in a processor having a large number ofpipeline stages.

Other features and advantages of the present invention will becomeapparent upon study of the remaining portions of the specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a pipelined microprocessoraccording to the present invention.

FIG. 2 is a block diagram of portions of the pipelined microprocessor ofFIG. 1 including a branch control apparatus according to the presentinvention.

FIG. 3 is a table illustrating two cache lines containing a branchinstruction that wraps across the two cache lines according to thepresent invention.

FIG. 4 is a flowchart illustrating operation of the branch controlapparatus of FIG. 2 according to the present invention.

FIGS. 5, 6, and 7 are timing diagrams illustrating examples of operationof the branch control apparatus of FIG. 2 according to the flowchart ofFIG. 4 according to the present invention.

FIG. 8 is a flowchart illustrating operation of the branch controlapparatus of FIG. 2 according to an alternate embodiment of the presentinvention.

FIG. 9 is a timing diagram illustrating an example of operation of thebranch control apparatus of FIG. 2 according to the flowchart of FIG. 8according to the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, a block diagram illustrating a pipelinedmicroprocessor 100 according to the present invention is shown. Theprocessor pipeline 100 includes a plurality of stages 101 through 132.In one embodiment, the microprocessor 100 comprises an x86 aarchitecture processor.

The first stage of the microprocessor 100 is the C-stage 101, orinstruction cache address generation stage. The C-stage 101 generates afetch address 162 that selects a cache line in an instruction cache 202(see FIG. 2).

The next stage is the I-stage 102, or instruction fetch stage. TheI-stage 102 is the stage where the processor 100 provides the fetchaddress 162 to the instruction cache 202 (see FIG. 2) in order to fetchinstructions for the processor 100 to execute. The instruction cache 202is described in more detail with respect to FIG. 2. In one embodiment,the instruction cache 202 is a two-cycle cache. A B-stage 104 is thesecond stage of the instruction cache 202 access. The instruction cache202 provides its data to a U-stage 106, where the data is latched in.The U-stage 106 provides the instruction cache data to a V-stage 108.

In the present invention, the processor 100 further comprises aspeculative branch target address cache (BTAC) 216 (see FIG. 2),described in detail below. The BTAC 216 is accessed in parallel with theinstruction cache 202 in the I-stage 102 using the instruction cache 202fetch address 162, thereby enabling relatively fast branching to reducebranch penalty. The BTAC 216 provides a speculative branch targetaddress 152 that is provided to the I-stage 102. The processor 100selectively chooses the target address 152 as the instruction cache 202fetch address to achieve a branch to the speculative target address 152.

Advantageously, as may be seen from FIG. 1, the branch target address152 supplied by the branch target address cache 216 in the U-stage 106enables the processor 100 to branch relatively early in the pipeline100, creating only a two-cycle instruction bubble. That is, if theprocessor 100 branches to the speculative target address 152, only twostages worth of instructions must be flushed. In other words, within twocycles, the target instructions of the branch will be available at theU-stage 106 in the typical case, i.e., if the target instructions arepresent in the instruction cache 202.

Advantageously, in most cases, the two-cycle bubble is small enough thatan instruction buffer 142, F-stage instruction queue 144 and/or X-stageinstruction queue 146, described below, may absorb the bubble.Consequently, in many cases, the speculative BTAC 216 enables theprocessor 100 to achieve zero-penalty branches.

The V-stage 108 is the stage in which instructions are written to theinstruction buffer 142. The instruction buffer 142 buffers instructionsfor provision to an F-stage 112. The instruction buffer 142 comprises aplurality of stages, or registers, for storing instruction bytesreceived from the instruction cache 202. In one embodiment, theinstruction buffer 142 is capable of buffering 128 instruction bytes. Inone embodiment, the instruction buffer 142 is similar to the instructionbuffer described in the U.S. patent application entitled APPARATUS ANDMETHOD FOR SELECTIVELY ACCESSING DISPARATE INSTRUCTION BUFFER STAGESBASED ON BRANCH TARGET ADDRESS CACHE HIT AND INSTRUCTION STAGE WRAP,incorporated by reference above. The V-stage 108 also includes decodelogic for providing information about the instruction bytes to theinstruction buffer 142, such as x86 prefix and mod R/M information, andwhether an instruction byte is a branch opcode value.

The F-stage 112, or instruction format stage 112, includes instructionformat logic 214 (see FIG. 2) for formatting instructions. Preferably,the processor 100 is an x86 processor, which allows for variable lengthinstructions in its instruction set. The instruction format logic 214receives a stream of instruction bytes from the instruction buffer 142and parses the stream into discrete groups of bytes constituting an x86instruction, and in particular providing the length of each instruction.

The F-stage 112 also includes branch instruction target addresscalculation logic for generating a non-speculative branch target address154 based on an instruction decode, rather than based speculatively onthe instruction cache 202 fetch address, like the BTAC 216 in theI-stage 102. The F-stage 112 non-speculative address 154 is provided tothe I-stage 102. The processor 100 selectively chooses the F-stage 112non-speculative address 154 as the instruction cache 202 fetch addressto achieve a branch to the non-speculative address 154.

An F-stage instruction queue 144 receives the formatted instructions.Formatted instructions are provided by the F-stage instruction queue 144to an instruction translator in the X-stage 114.

The X-stage 114, or translation stage 114, instruction translatortranslates x86 macroinstructions into microinstructions that areexecutable by the remainder of the pipeline stages. The translatedmicroinstructions are provided by the X-stage 114 to an X-stageinstruction queue 146.

The X-stage instruction queue 146 provides translated microinstructionsto an R-stage 116, or register stage 116. The R-stage 116 includes theuser-visible x86 register set, in addition to other non-user-visibleregisters. Instruction operands for the translated microinstructions arestored in the R-stage 116 registers for execution of themicroinstructions by subsequent stages of the pipeline 100.

An A-stage 118, or address stage 118, includes address generation logicthat receives operands and microinstructions from the R-stage 116 andgenerates addresses required by the microinstructions, such as memoryaddresses for load/store microinstructions.

A D-stage 122, or data stage 122, includes logic for accessing dataspecified by the addresses generated by the A-stage 118. In particular,the D-stage 122 includes a data cache for caching data within theprocessor 100 from a system memory. In one embodiment, the data cache isa two-cycle cache. The D-stage 122 provides the data cache data to anE-stage 126.

The E-stage 126, or execution stage 126, includes execution logic, suchas arithmetic logic units, for executing the microinstructions based onthe data and operands provided from previous stages. In particular, theE-stage 126 produces a resolved target address 156 of all branchinstructions. That is, the E-stage 126 target address 156 is known to bethe correct target address of all branch instructions with which allpredicted target addresses must match. In addition, the E-stage 126produces a resolved direction for all branch instructions, i.e., whetherthe branch is taken or not taken.

An S-stage 128, or store stage 128, performs a store to memory of theresults of the microinstruction execution received from the E-stage 126.In addition, the target address 156 of branch instructions calculated inthe E-stage 126 is provided to the instruction cache 202 in the I-stage102 from the S-stage 128. Furthermore, the BTAC 216 of the I-stage 102is updated from the S-stage 128 with the resolved target addresses ofbranch instructions executed by the pipeline 100 for caching in the BTAC216. In addition, other speculative branch information (SBI) 236 (seeFIG. 2) is updated in the BTAC 216 from the S-stage 128. The speculativebranch information 236 includes the branch instruction length, thelocation within an instruction cache 202 line of the branch instruction,whether the branch instruction wraps over multiple instruction cache 202lines, whether the branch is a call or return instruction, andinformation used to predict the direction of the branch instruction.

A W-stage 132, or write-back stage 132, writes back the result from theS-stage 128 into the R-stage 116 registers, thereby updating theprocessor 100 state.

The instruction buffer 142, F-stage instruction queue 144 and X-stageinstruction queue 146, among other things, serve to minimize the impactof branches upon the clocks per instruction value of the processor 100.

Referring now to FIG. 2, a block diagram of portions of the pipelinedmicroprocessor 100 of FIG. 1 including a branch control apparatusaccording to the present invention is shown.

The microprocessor 100 includes an instruction cache 202 that cachesinstruction bytes. The instruction cache 202 comprises an array of cachelines for storing instruction bytes. The array of cache lines is indexedby a fetch address 162 of FIG. 1. That is, the fetch address 162 selectsone of the cache lines in the array. The instruction cache 202 providesthe selected cache line of instruction bytes to the instruction buffer142 of FIG. 1 via a data bus 242.

In one embodiment, the instruction cache 202 comprises a 64KB 4-way setassociative cache, with 32-byte cache lines per way. In one embodiment,one half of the selected cache line of instruction bytes is provided bythe instruction cache 202 at a time, i.e., 16 bytes are provided duringtwo separate periods each. In one embodiment, the instruction cache 202is similar to an instruction cache described in U.S. patent applicationSer. No. 09/849,736 entitled SPECULATIVE BRANCH TARGET ADDRESS CACHE,having a common assignee, and which is hereby incorporated by referencein its entirety for all purposes. The instruction cache 202 generates atrue value on a MISS signal 204 if the fetch address 162 misses in theinstruction cache 202.

The microprocessor 100 also includes a bus interface unit (BIU) 206 thatfetches cache lines from a memory via a data bus 266. In particular, theBIU 206 fetches cache lines from the memory if the instruction cache 202generates a true value on MISS signal 204. The instruction cache 202also provides the MISS signal 204 to the BIU 206.

The microprocessor 100 also includes a response buffer 208. The responsebuffer 208 receives caches lines from the BIU 206. The response buffer208 also receives cache lines from a level-2 cache via data bus 212. Theresponse buffer 208 provides cache lines of instruction bytes to theinstruction buffer 142 via a data bus 244. When the response buffer 208has a cache line of instruction bytes to provide to the instructionbuffer 142, the response buffer 208 generates a true value on an RBRDYsignal 238.

When a cache line is stored into the instruction buffer 142, either fromthe instruction cache 202 or from the response buffer 208, such that theinstruction buffer 142 becomes full, the instruction buffer 142generates a true value on a FULL signal 246 to indicate that it cannotpresently accept instruction bytes.

The microprocessor 100 also includes instruction format logic 214. Theinstruction format logic 214 receives instruction bytes from theinstruction buffer 142. The instruction format logic 214 formats, orparses, the instruction bytes received into an instruction. Inparticular, the instruction format logic 214 determines the size inbytes of the instruction. The instruction format logic 214 provides thelength of the currently formatted instruction via instruction lengthsignal 248. The instruction format logic 214 provides the formattedinstruction to the remainder of the microprocessor 100 pipeline forfurther decode and execution. In one embodiment, the instruction formatlogic 214 is capable of formatting multiple instructions permicroprocessor 100 clock cycle.

The microprocessor 100 also includes a branch target address cache(BTAC) 216. The BTAC 216 also receives the instruction cache 202 fetchaddress 162. The BTAC 216 comprises an array of storage elements forcaching fetch addresses of previously executed branch instructions andtheir associated branch target addresses. The storage elements alsostore other speculative branch information related to the branchinstructions for which the target addresses are cached. In particular,the storage elements store an indication of whether the multi-bytebranch instructions wrap across two instruction cache lines. The fetchaddress 162 indexes the array of storage elements in the BTAC 216 toselect one of the storage elements.

The BTAC 216 outputs the target address 152 of FIG. 1 and speculativebranch information (SBI) 236 from the storage element selected by thefetch address 162. In one embodiment, the SBI 236 includes the branchinstruction length, the location of the branch instruction in the cacheline, whether the branch is a call or return instruction, and aprediction of whether the branch instruction will be taken or not taken.

The BTAC 216 also outputs a HIT signal 234 that indicates whether thefetch address 162 hit in the BTAC 216. In one embodiment, the BTAC 216is similar to a BTAC described in U.S. Patent application entitledSPECULATIVE BRANCH TARGET ADDRESS CACHE, which is incorporated byreference above. In one embodiment, the BTAC 216 is a speculative BTACbecause the microprocessor 100 branches to the target address 152provided by the BTAC 216 before the instruction cache line provided bythe instruction cache 202 is decoded to know whether or not a branchinstruction is even present in the cache line selected by the fetchaddress. That is, the microprocessor 100 speculatively branches eventhough the possibility exists that no branch instruction is present inthe cache line selected by the fetch address hitting in the BTAC 216.

The BTAC 216 also outputs a WRAP signal 286, which specifies whether thebranch instruction wraps across two cache lines. The WRAP signal 286value is cached in the BTAC 216 along with the branch instruction targetaddress after execution of the branch instruction.

Referring now to FIG. 3, a table illustrating two cache lines containinga branch instruction that wraps across the two cache lines is shown. Thetable shows a first cache line, denoted cache line A 302 whose lastinstruction byte contains an opcode byte for an x86 JCC (conditionaljump) instruction. The table also shows a second cache line, denotedcache line B 304 whose first instruction byte contains a signeddisplacement byte (disp) for the JCC instruction. Whenever themicroprocessor 100 executes a branch instruction and caches the fetchaddress of the cache line containing the branch instruction in the BTAC216 along with the target address of the branch instruction, themicroprocessor 100 also caches an indicator of whether the branchinstruction wraps across two cache lines, like the JCC instruction ofFIG. 3. If the fetch address subsequently hits in the BTAC 216, the BTAC216 provides the cached wrap indicator on the WRAP signal 286. The wrapindicator enables the branch control apparatus to know that the fetchaddress of both cache lines must be provided to the instruction cache202 in order to obtain all the instruction bytes for the branchinstruction.

Referring again to FIG. 2, the microprocessor 100 also includes controllogic 222. The HIT signal 234, the SBI 236, the WRAP signal 286, theMISS signal 204, the FULL signal 246, the RBRDY signal 238, and theinstruction length signal 248 are all provided as inputs to the controllogic 222. The operation of the control logic 222 is described in moredetail below.

The microprocessor 100 also includes a mux 218. The mux 218 receives atleast six addresses as inputs and selects one of the inputs as the fetchaddress 162 to the instruction cache 202 in response to a control signal168 generated by the control logic 222. The mux 218 receives the targetaddress 152 from the BTAC 216. The mux 218 also receives a nextsequential fetch address 262. The next sequential fetch address 262 isthe previous fetch address incremented by the size of an instructioncache 202 cache line by an incrementer 224. The incrementer 224 receivesthe fetch address 162 and provides the next sequential fetch address 262to the mux 218.

The mux 218 also receives the resolved target address 156 of FIG. 1. Theresolved target address 156 is provided by execution logic in themicroprocessor 100. The execution logic calculates the resolved targetaddress 156 based execution of a branch instruction. If after branchingto the target address 152 provided by the BTAC 216, the microprocessor100 later determines that the branch was erroneous, the microprocessor100 corrects the error by flushing the pipeline and branching to eitherthe resolved target address 156 or to the fetch address of a cache lineincluding the instruction following the branch instruction. In oneembodiment, the microprocessor 100 corrects the error by flushing thepipeline and branching to the fetch address of a cache line includingthe branch instruction itself, if the microprocessor 100 determines thatno branch instruction was present in the cache line as presumed. Theerror correction is as described in U.S. patent application Ser. No.09/849,658 entitled APPARATUS, SYSTEM AND METHOD FOR DETECTING ANDCORRECTING ERRONEOUS SPECULATIVE BRANCH TARGET ADDRESS CACHE BRANCHES,having a common assignee, and which is hereby incorporated by referencein its entirety for all purposes.

In one embodiment, the mux 218 also receives the non-speculative targetaddress 154 of FIG. 1. The non-speculative target address 154 isgenerated by other branch prediction elements, such as a call/returnstack and a branch target buffer (BTB) that caches target addresses ofindirect branch instructions based on the branch instruction pointer.The mux 218 selectively overrides the target address 152 provided by theBTAC 216 with the non-speculative target address 154 as described inU.S. patent application Ser. No. 09/849,799 entitled SPECULATIVE BRANCHTARGET ADDRESS CACHE WITH SELECTIVE OVERRIDE BY SECONDARY PREDICTORBASED ON BRANCH INSTRUCTION TYPE, having a common assignee, and which ishereby incorporated by reference in its entirety for all purposes.

The mux 218 also receives a backup fetch address 274. The microprocessor100 includes a fetch address register file 282 that provides the backupfetch address 274 to the mux 218. In one embodiment of themicroprocessor 100, stages C 101 through V 108 cannot stall. That is,all of the state is not saved for these stages on each clock cycle.Consequently, if a cache line reaches the instruction buffer 142 and theinstruction buffer 142 is full, the cache line is lost. If theinstruction buffer 142 is relatively large, it may be advantageous tosave complexity and space in the microprocessor 100 by not having thestate saving logic.

Although the upper stages of the pipeline 100 may not stall, the fetchaddress of a cache line that is lost due to a full instruction buffer142 is saved in the fetch address register file 282 and provided to themux 218 as the backup fetch address 274. As cache lines flow down thepre-decode pipeline stages of the microprocessor 100, the correspondingfetch address 152, provided by the mux 218, flows down the fetch addressregister file 282. Use of the backup fetch address 274 will be describedin more detail below with respect to the remaining figures.

The mux 218 also receives a saved target address 284. The saved targetaddress 284 is a previous value of the target address 152 output by theBTAC 216. The saved target address 284 is saved in a save register 228.The save register 228 receives the output of a save mux 226. The savemux 226 receives the BTAC 216 target address 152. The save mux 226 alsoreceives the output of the save register 228 for holding the value ofthe saved target address 284. The save mux 226 is controlled by acontrol signal 276 generated by the control logic 222.

The microprocessor 100 also includes a flag register 232. The controllogic 222 sets the flag register 232 to a true value whenever a wrappedBTAC 216 branch instruction is pending. That is, the flag register 232indicates that the save register 228 currently stores a BTAC 216 targetaddress 152 for a branch instruction that wraps across two cache lines.

Referring now to FIG. 4, a flowchart illustrating operation of thebranch control apparatus of FIG. 2 according to the present invention isshown. In the following description of FIG. 4, fetch address A refers toa fetch address of a cache line A that contains at least a first portionof a branch instruction, such as cache line A 302 of FIG. 3. Fetchaddress B refers to a fetch address of a cache line B that contains asecond portion of a wrapping branch instruction, such as cache line B304 of FIG. 3. Cache line T refers to a cache line that contains one ormore target instructions of a branch instruction selected by a targetaddress of the branch instruction. For clarity and simplicity, theflowchart of FIG. 4 assumes that both cache line A and cache line T hitin the instruction cache 202 of FIG. 2. The flowchart of FIG. 4specifies operation if cache line B hits in the instruction cache 202and specifies operation if cache line B does not hit in the instructioncache 202. Flow begins at block 402.

At block 402, mux 218 of FIG. 2 applies fetch address A as the fetchaddress 162 to the instruction cache 202 and to the BTAC 216 of FIG. 2.In the typical case, program flow proceeds sequentially, hence, mux 218selects the next sequential fetch address 262 of FIG. 2 as fetch addressA. Flow proceeds from block 402 to block 404.

At block 404, the instruction cache 202 provides line A on data bus 242of FIG. 2 in response to the application of fetch address A during step402. Line A contains at least a first portion of a branch instruction,and fetch address A is cached in the BTAC 216. Whether cache line Acontains all or part of the branch instruction will be determined atdecision block 408 described below. The BTAC 216 provides a targetaddress 152 of FIG. 1 for the cached branch instruction in response tofetch address A. Flow proceeds from block 404 to block 406.

At block 406, the target address 152 provided by the BTAC 216 duringstep 404 is stored in the save register 228 of FIG. 2. That is, controllogic 222 controls save mux 226 of FIG. 2 to select target address 152from the BTAC 216 for storage in the save register 228 because a BTAC216 hit occurred, as indicated on HIT signal 234 of FIG. 2. Upon storingthe target address 152 into the save register 228, the control logic 222sets the flag register 232 to a true value. Flow proceeds from block 406to decision block 408.

At decision block 408, control logic 222 of FIG. 2 determines whetherthe branch instruction wraps beyond cache line A, i.e., across two cachelines. In particular, control logic 222 examines the WRAP signal 286 ofFIG. 2 to determine if it has a true value. If not, then the branchinstruction is wholly contained in cache line A, and flow proceeds toblock 412. Otherwise, the first part of the branch instruction iscontained in cache line A, the second part of the branch instruction iscontained in cache line B, and flow proceeds to block 428.

At block 412, the target address 152 provided by the BTAC 216 duringstep 404 is selected by mux 218 and applied as fetch address 162 to theinstruction cache 202. If flow reaches block 412, then the BTAC 216branch instruction is not a wrapping branch instruction. Hence, thetarget address 152 is applied after fetch address A, since it would beincorrect to apply fetch address B to the instruction cache 202, sincethe entire branch instruction is contained in cache line A. Flowproceeds from block 412 to block 414.

At block 414, cache line A is stored in the instruction buffer 142 ofFIG. 2. Flow proceeds from block 414 to block 416.

At block 416, the instruction cache 202 provides cache line T, whichcontains the target instructions of the branch instruction. Theinstruction cache 202 provides cache line T in response to the targetaddress 152 applied to the instruction cache 202 during step 412. Flowproceeds from block 416 to decision block 418.

At decision block 418, control logic 222 determines whether theinstruction buffer 142 is full. In particular, the control logic 222examines the value of the FULL signal 246 of FIG. 2 generated by theinstruction buffer 142 to see if it is true. If not, flow proceeds toblock 422. Otherwise, flow proceeds to block 424.

At block 422, cache line T is stored in the instruction buffer 142. Atthis point, the branch instruction and its target instructions arestored in the instruction buffer 142 so that they can be formatted bythe instruction format logic 214 of FIG. 2. Upon storing cache line Tinto the instruction buffer 142, the control logic 222 sets the flagregister 232 to a false value. If the branch instruction was anon-wrapping branch, i.e., if flow proceeded from decision block 408 toblock 412, then the instruction buffer 142 contains a cache line Acontaining the entire branch instruction, and cache line T, containingthe target instructions. However, if the branch instruction was awrapping branch, i.e., if flow proceeded from decision block 408 toblock 428, then the instruction buffer 142 will contain cache line Acontaining the first portion of the branch instruction, cache line B,containing the second portion of the branch instruction, and cache lineT, containing the target instructions, as described below. Flow ends atblock 422.

At block 424, control logic 222 waits for the instruction buffer 142 tobecome not full. That is, control logic 222 examines the FULL signal 246until it becomes false. While the control logic 222 is waiting for theFULL signal 246 to become false, the saved target address 284 continuesto be held in the save register 228. Flow proceeds from block 424 toblock 426.

At block 426, mux 218 selects the saved target address 284 provided bythe save register 228 and applies the save target address 284 as fetchaddress 162 to the instruction cache 202. The saved target address 284was stored in the save register 228 during step 406. If flow reachesblock 426 from block 454 described below, then the BTAC 216 branchinstruction is a wrapping branch instruction. In this case, the targetaddress 152 is applied after fetch address B so that the entire branchinstruction is stored in the instruction buffer 142 prior to the branchtarget instructions in cache line T being stored in the instructionbuffer 142. Flow proceeds from block 426 to block 416.

At block 428, cache line A is stored in the instruction buffer 142. Inthis case, cache line A contains only the first portion of the wrappingbranch instruction, not the entire branch instruction. Flow proceedsfrom block 428 to block 432.

At block 432, mux 218 selects the next sequential fetch address 262provided by the incrementer 224 of FIG. 2, which will be fetch addressB, and applies fetch address B as the fetch address 162 to theinstruction cache 202. It is necessary to apply fetch address B in orderto obtain cache line B, which contains the second portion of thewrapping branch instruction, so that all the instruction bytes of thebranch instruction may be stored in the instruction buffer for decoding.Flow proceeds from block 432 to decision block 434.

At decision block 434, control logic 222 and BIU 206 of FIG. 2 determinewhether fetch address B hit in the instruction cache 202. In particular,control logic 222 and bus BIU 206 examine the MISS signal 204 of FIG. 2generated by the instruction cache 202 to determine if the value istrue. If not, flow proceeds to block 436. Otherwise, flow proceeds toblock 444.

At block 436, either the BIU 206 fetches cache line B from memory, orcache line B is provided by the level-2 cache. When cache line B arrivesin response buffer 208 of FIG. 2, the response buffer 208 generates atrue value on the RBRDY signal 238 to notify control logic 222 thatcache line B is available. Flow proceeds from block 436 to block 438.

At block 438, cache line B is stored in the instruction buffer 142 fromthe response buffer 208. Flow proceeds from block 438 to block 442.

At block 442, mux 218 selects the next sequential fetch address 262provided by the incrementer 224 and applies the next sequential fetchaddress as the fetch address 162 to the instruction cache 202. That is,if cache line B is not present in the instruction cache 202, thiscondition is treated as a BTAC 216 miss. If the E-stage 126 of FIG. 1later determines that the branch instruction is taken, the mispredictionwill be corrected by branching to the resolved target address 156. Theembodiment of FIG. 4 has the advantage of requiring less control logicthan the embodiment of FIG. 8, described below, which handles the caseof a wrapping BTAC 216 branch, whose second cache line misses in theinstruction cache 202. In a microprocessor 100 in which the probabilityis very low that a branch instruction will wrap and generate aninstruction cache 202 miss for its second portion, the embodiment ofFIG. 4 is advantageous because it requires less complexity. Flow ends atblock 442.

At block 444, the instruction cache 202 provides cache line B on databus 242 in response to the application of fetch address B during step432. Line B contains the second portion of the branch instruction. Flowproceeds from block 444 to decision block 446.

At decision block 446, control logic 222 determines whether theinstruction buffer 142 is full by examining the value of the FULL signal246 to see if it is true. That is, the control logic 222 determineswhether the store of cache line A into the instruction buffer 142 duringstep 428 filled the instruction buffer 142. If so, flow proceeds toblock 448. If not, flow proceeds to block 454.

At block 448, control logic 222 waits for the instruction buffer 142 tobecome not full. That is, control logic 222 examines the FULL signal 246until it becomes false. Flow proceeds from block 448 to block 452.

At block 452 mux 218 selects the backup fetch address 274 of FIG. 2provided by the fetch address register file 282 of FIG. 2, which will befetch address B, and applies fetch address B as the fetch address 162 tothe instruction cache 202. It is necessary to apply fetch address B inorder to obtain cache line B, which contains the second portion of thewrapping branch instruction. Flow proceeds from block 452 to block 434to determine whether the application of the backup fetch address B hitsin the instruction cache 202.

At block 454, cache line B is stored in the instruction buffer 142.Cache line B contains the second portion of the wrapping branchinstruction. Flow proceeds from block 454 to block 426 to get cache lineT, which contains the branch target instructions, into the instructionbuffer 142.

As may be seen from the flowchart of FIG. 4, the present inventionprovides an improvement over a solution to the wrapping BTAC 216 branchproblem that simply treats all wrapping BTAC 216 branches as BTAC 216misses. The percentage of BTAC 216 branches that wrap is non-negligible,and the present invention provides a means of branching rather than notbranching and having to correct, thereby potentially saving many clockcycles. This is particularly beneficial in a microprocessor 100 in whichthe number of stages is relatively large.

Referring now generally to FIGS. 5, 6, 7, and 9, timing diagramsillustrating examples of operation of the branch control apparatus ofFIG. 2 according to the present invention are shown. FIGS. 5, 6, and 7illustrate operation according to the flowchart of FIG. 4, whereas, FIG.9 illustrates operation according to the alternate embodiment flowchartof FIG. 8 described below. The timing diagrams comprise a matrix ofcells having 7 rows and 33 columns. The first column, beginning with thesecond row, is denoted C, I, B, U, V, and F, corresponding to theC-stage 101, I-stage 102, B-stage 104, U-stage 106, V-stage 108, andF-stage 112 of FIG. 1. The first row, beginning with the second column,is denoted 1 through 32, corresponding to 32 clock cycles of themicroprocessor 100.

Each of the cells in the matrix specifies the contents of the specifiedstage during the specified clock cycle. For clarity and simplicity, eachof the cells is denoted herein as (s,c), where s is the stage, and c isthe clock cycle. For example, cell (V,5) denotes the contents of theV-stage 108 during clock cycle 5. The cells are either blank, or haveone of four letters A, B, C, or T in them. The letter A designateseither fetch address A or cache line A, of FIG. 4, or both dependingupon the context of the stage. Similarly, the letter B designates eitherfetch address B or cache line B, of FIG. 4, or both, and the letter Tdesignates either a BTAC 216 target address of a branch instruction orcache line T, of FIG. 4, or both. The letter C designates either thenext sequential fetch address after fetch address B or the nextsequential cache line after cache line B or both. For example, in FIG.5, the cell corresponding to the contents of the I-stage 102 duringclock cycle 2, denoted (I,2), contains an A, to signify that the I-stage102 receives fetch address A during clock cycle 2. That is, address A isapplied as the fetch address 162 of FIG. 1 to the instruction cache 202of FIG. 2 as described with respect to block 402 of FIG. 4.

In addition, below the matrix, the values of the WRAP signal 286, theFULL signal 246, the MISS signal 204, and RBRDY signal 238 of FIG. 2during each of the 32 clock cycles is shown. A polarity convention ischosen for illustration purposes such that if the signal is low, thenthe value is false. For example, if the FULL signal 246 is low, thevalue is false, i.e., the instruction buffer 142 of FIG. 1 is not full;conversely, if the FULL signal 246 is high, the value is true, i.e., theinstruction buffer 142 of FIG. 1 is full. However, the invention issusceptible to use of either polarity convention. References to blocknumbers, such as block 412, are to blocks of the flowchart of FIG. 4.

Referring now to FIG. 5, a timing diagram is shown illustrating anexample of operation of the microprocessor 100 of FIG. 1 according tothe flowchart of FIG. 4 in the case of a non-wrapping BTAC 216 branch,wherein the instruction buffer 142 is full when cache line T isinitially ready for storage in the instruction buffer 142. Hence, theWRAP signal 286, the MISS signal 204, and the RBRDY signal 238 are falsethroughout the clock cycles of FIG. 5, and the FULL signal 246 is trueduring a portion of the clock cycles, in particular clock cycles 6through 10.

In cell (C,1), mux 218 of FIG. 2 selects fetch address A as fetchaddress 162. In cell (I,2), the I-stage 102 applies fetch address A tothe instruction cache 202 and to the BTAC 216, according to block 402.In cell (B,3), the instruction cache 202 is selecting cache line A,during its second access cycle. In cell (U,4), the instruction cache 202provides cache line A, according to block 404.

In cell (V,5), cache line A is written to the instruction buffer 142,according to block 414. In the example of FIG. 5, storing cache line Ain the instruction buffer 142 causes the instruction buffer 142 to befull. Hence, during clock 6, the FULL signal 246 is true. In the exampleof FIG. 5, the FULL signal 246 remains true until clock 11.

In cell (C,4), mux 218 selects the target address 152 provided by theBTAC 216 during block 404 as the fetch address 162. In cell (I,5), thetarget address 152 is applied to the instruction cache 202, according toblock 412, since the WRAP signal 286 is false in the example.

In cells (V,6) through (V,14), cache line A remains in the instructionbuffer 142 and is not provided to the instruction format logic 214because the instruction format logic 214 is formatting otherinstructions ahead of cache line A. An example of a cause of theinstruction buffer 142 remaining full for several clock cycles is whereone or more instructions which require a large number of clock cycles toexecute, such as floating point divides, are being executed in thepipeline. These instructions cause the stages of the pipeline 100 abovethe execution stage 126 to stall.

In cell (B,6), the instruction cache 202 is selecting cache line T,during its second access cycle. In cell (U,7), the instruction cache 202provides cache line T, according to block 416. However, during clockcycle 7 the instruction buffer 142 is full, as determined during block418. Hence, during clock cycle 8, cache line T is lost since theinstruction buffer 142 cannot accept cache line T since the instructionbuffer 142 is full. Control logic 222 of FIG. 2 waits until the FULLsignal 246 is false, according to block 424.

In cell (C,11), mux 218 selects the saved target address 284 provided bythe save register 228 as the fetch address 162, since the control logic222 determined that the FULL signal 246 is now false in clock cycle 11.In cell (I,12), the saved target address 284 is applied to theinstruction cache 202, according to block 426. In cell (B,13), theinstruction cache 202 is selecting cache line T, during its secondaccess cycle. In cell (U,14), the instruction cache 202 provides cacheline T, according to block 416.

In cell (F,15), cache line A proceeds to the instruction format logic114 where the branch instruction is formatted. In cell (V,15), cacheline T is written to the instruction buffer 142, according to block 422,since the instruction buffer 142 is no longer full, as determined duringblock 418. In cell (F,16), cache line T proceeds to the instructionformat logic 114 where the branch target instruction is formatted.

Referring now to FIG. 6, a timing diagram, similar to FIG. 5,illustrating a second example of operation of the branch controlapparatus of FIG. 2 according to the flowchart of FIG. 4 according tothe present invention is shown. FIG. 6 illustrates an example ofoperation of the microprocessor 100 of FIG. 1 according to the flowchartof FIG. 4 in the case of a wrapping BTAC 216 branch, wherein the secondportion of the branch instruction, contained in cache line B, misses inthe instruction cache 102. Hence, the FULL signal 246 is falsethroughout the clock cycles of FIG. 6, and the WRAP signal 286, the MISSsignal 204, and the RBRDY signal 238 are true during a portion of theclock cycles, in particular during clock cycles 4, 5, and 24,respectively.

Cells (C,1), (I,2), (B,3), (U,4), and (V,5) are similar to correspondingcells of FIG. 5, with fetch address A and cache line A proceeding downthe upper stages of the microprocessor 100 pipeline. During clock cycle4, the WRAP signal 286 is true, specifying that the BTAC 216 indicatedthe branch instruction wraps across cache lines A and B. In cell (F,6),cache line A proceeds to the F-stage 112.

In cell (C,2), mux 218 selects the next sequential fetch address 262,which is fetch address B, as the fetch address 162 since the controllogic 222 determined that the branch instruction is a wrapping BTAC 216branch, according to block 408. In cell (I,3), fetch address B isapplied to the instruction cache 202, according to block 432, since theWRAP signal 286 is true in the example. In cell (B,4), the instructioncache 202 is selecting cache line B, during its second access cycle.However, during clock 5, the instruction cache 102 determines that fetchaddress B is a miss, and accordingly asserts the MISS signal 204.Consequently, the instruction cache 102 is unable to provide cache lineB.

During clock cycles 7 through 23, the microprocessor 100 waits for cacheline B to be fetched from memory into the response buffer 208, accordingto block 436. During clock 24, the response buffer 208 of FIG. 2 assertsthe RBRDY signal 238 when cache line B arrives. In cell (V,24), cacheline B is stored into the instruction buffer 142 from the responsebuffer 208, according to block 438. In cell (F,25), cache line Bproceeds to the F-stage 112.

In cell (C,25), mux 218 selects the next sequential fetch address 262,which is fetch address C, as the fetch address 162, according to block442, since the control logic 222 determined that cache line B missed inthe instruction cache 102. Hence, the microprocessor 100 treats the caseof FIG. 6 as a BTAC 216 miss by not branching to the target address 132provided by the BTAC 216, but instead fetching the next sequentialinstruction. In cell (I,26), the I-stage 102 applies fetch address C tothe instruction cache 202. In cell (B,27), the instruction cache 202 isselecting cache line C, during its second access cycle. In cell (U,28),the instruction cache 202 provides cache line C. In cell (V,29), cacheline C is written to the instruction buffer 142. In cell (F,30), cacheline C proceeds to the F-stage 112.

Referring now to FIG. 7, a timing diagram, similar to FIG. 5,illustrating a third example of operation of the branch controlapparatus of FIG. 2 according to the flowchart of FIG. 4 according tothe present invention is shown. FIG. 7 illustrates an example ofoperation of the microprocessor 100 of FIG. 1 according to the flowchartof FIG. 4 in the case of a wrapping BTAC 216 branch, wherein cache lineA fills the instruction buffer 142. Hence, the MISS signal 204 and theRBRDY signal 238 are false throughout the clock cycles of FIG. 5, andthe WRAP signal 286 and the FULL signal 246 is true during a portion ofthe clock cycles. In particular, the WRAP signal 286 is true duringclock cycle 4, and the FULL signal 246 is true during clock cycles 6through 10.

Cells (C,1), (I,2), (B,3), (U,4), (V,5) through (V,14), and (F,15) aresimilar to corresponding cells of FIG. 5, with fetch address A and cacheline A proceeding down the upper stages of the microprocessor 100pipeline to the F-stage 112. During clock cycle 4, the WRAP signal 286is true, specifying that the BTAC 216 indicated the branch instructionwraps across cache lines A and B.

Cells (C,2), (I,3), and (B,4) are similar to corresponding cells of FIG.6, with fetch address B and cache line B proceeding down the C, I, and Bstages of the microprocessor 100 pipeline. In cell (U,5), theinstruction cache 102 provides cache line B, according to block 444,since fetch address B hit in the instruction cache 102.

However, during clock 6, the instruction buffer 142 asserts the FULLsignal 246 because cache line A has filled the instruction buffer 142.Consequently, the control logic 222 waits for the FULL signal 246 signalto become false, according to block 448, which occurs in clock cycle 11.

In cell (C,11), mux 218 selects the backup fetch address 274 from thefetch address register file 282, which is fetch address B, in responseto the FULL signal 246 becoming false. In cell (I,12), fetch address Bis applied to the instruction cache 102, according to block 452. In cell(B,13), the instruction cache 202 is selecting cache line B, during itssecond access cycle. In cell (U,14), the instruction cache 202 providescache line B, according to block 444, since fetch address B hits in theinstruction cache 102. In cell (V,15), cache line B is written to theinstruction buffer 142, according to block 454, since the instructionbuffer 142 is not full. In cell (F,16), cache line B progresses to theF-stage 112.

In cell (C,12), mux 218 selects the saved target address 284 from saveregister 228. In cell (I,13), the saved target address 284 is applied tothe instruction cache 102, according to block 426. In cell (B,14), theinstruction cache 202 is selecting cache line T, during its secondaccess cycle. In cell (U,15), the instruction cache 202 provides cacheline T, according to block 416. In cell (V,16), cache line T is writtento the instruction buffer 142, according to block 422, since theinstruction buffer 142 is not full. In cell (F,17), cache line Tprogresses to the F-stage 112.

Referring now to FIG. 8, a flowchart illustrating operation of thebranch control apparatus of FIG. 2 according to an alternate embodimentof the present invention is shown. The flowchart of FIG. 8 is identicalto the flowchart of FIG. 4 with the exception that FIG. 8 does notinclude blocks 438 and 442. Instead, flow proceeds from block 436 todecision block 446. That is, rather than treating a miss of fetchaddress B in the instruction cache 102 as a BTAC 216 miss, theembodiment of FIG. 8 handles the condition. The embodiment handles thecondition by backing up to fetch address B after the instruction buffer142 is no longer full, and subsequently applying the saved targetaddress 284 to obtain cache line T, as will be illustrated with respectto FIG. 9.

Referring now to FIG. 9, a timing diagram, similar to FIG. 6,illustrating an example of operation of the branch control apparatus ofFIG. 2 according to the flowchart of FIG. 8 according to the presentinvention is shown. FIG. 9 illustrates an example of operation of themicroprocessor 100 of FIG. 1 according to the flowchart of FIG. 8 in thecase of a wrapping BTAC 216 branch, wherein cache line B, which containsthe second portion of the branch instruction, misses in the instructioncache 102, and cache line A fills the instruction buffer 142. Clockcycles 1 through 23 of FIG. 9 are the same as corresponding ones of FIG.6, except that the FULL signal 246 is true during clock cycles 6 through25 in FIG. 9.

When cache line B arrives in the response buffer 208 during clock cycle24, the instruction buffer 142 is full, as determined according to block446. Hence, cache line B is not written into the instruction buffer 142,but is written into the instruction cache 102.

In the example, the FULL signal 246 goes false during clock cycle 26, asdetermined during block 448. Hence, in cell (C,26), mux 218 selectsbackup fetch address 274 as fetch address 152. In cell (I,27), thebackup fetch address 274 is applied to the instruction cache 102,according to block 452. In cell (B,28), the instruction cache 202 isselecting cache line B, during its second access cycle. In cell (U,29),the instruction cache 202 provides cache line B, according to block 444.Cache line B was previously written into the instruction cache 102 fromthe response buffer 208, during clock cycle 25. In cell (V,30), cacheline B is written to the instruction buffer 142, according to block 454,since the instruction buffer 142 is not full. In cell (F,31), cache lineB progresses to the F-stage 112.

In cell (C,27), mux 218 selects the saved target address 284 as fetchaddress 152. In cell (I,28), the saved target address 284 is applied tothe instruction cache 102, according to block 426. In cell (B,29), theinstruction cache 202 is selecting cache line T, during its secondaccess cycle. In cell (U,30), the instruction cache 202 provides cacheline T, according to block 416. In cell (V,31), cache line T is writtento the instruction buffer 142, according to block 422, since theinstruction buffer 142 is not full. In cell (F,32), cache line Tprogresses to the F-stage 112.

As may be observed from FIGS. 8 and 9, the alternate embodiment has theadvantage of not incurring the additional clock cycles associated withcorrecting a mispredicted taken branch, i.e., a BTAC 216 hit that istreated as a BTAC 216 miss because it wraps and the second cache linecontaining the second part of the branch misses in the instruction cache202. Rather, as may be observed from FIG. 9, the BTAC 216 target address152 is supplied to the instruction cache 202 at the earliest clock cyclepossible after fetch address B.

Although the present invention and its objects, features, and advantageshave been described in detail, other embodiments are encompassed by theinvention. For example, the number and arrangement of stages in thepipeline may vary. The size and construction of the BTAC, instructioncache, or instruction buffer may vary. The size of a cache line mayvary.

Those skilled in the art should appreciate that they can readily use thedisclosed conception and specific embodiments as a basis for designingor modifying other structures for carrying out the same purposes of thepresent invention without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A branch control apparatus in a microprocessor having an instructioncache, coupled to an address bus, for providing cache lines to aninstruction buffer, the apparatus comprising: a target address of abranch instruction, said target address provided by a branch targetaddress cache (BTAC); a wrap signal, originating directly from saidBTAC, for indicating whether said branch instruction wraps across firstand second cache lines of said instruction cache; and an addressregister, coupled to said BTAC, for storing said target address; whereinif said wrap signal indicates said branch instruction wraps across saidfirst and second cache lines, said address register provides said targetaddress on the address bus to the instruction cache to select a thirdcache line, said third cache line containing a target instruction ofsaid branch instruction, wherein said address register provides saidtarget address to the instruction cache if said second cache line hitsin the instruction cache.
 2. The branch control apparatus of claim 1,wherein said address register provides said target address on theaddress bus after a fetch address for said second cache line is providedon the address bus.
 3. The branch control apparatus of claim 2, whereinsaid BTAC is coupled to the address bus, wherein said BTAC provides saidtarget address in response to a fetch address provided on the addressbus.
 4. The branch control apparatus of claim 3, wherein said BTACprovides said target address in response to a fetch address of saidfirst cache line.
 5. The branch control apparatus of claim 4, whereinsaid first cache line contains a first portion of said branchinstruction.
 6. The branch control apparatus of claim 2, furthercomprising: a full signal, coupled to the instruction buffer, forindicating whether the instruction buffer is full.
 7. The branch controlapparatus of claim 6, wherein said address register provides said targetaddress to the instruction cache only if said full signal indicates theinstruction buffer is not full.
 8. The branch control apparatus of claim6, further comprising: a backup register, coupled to the address bus,for storing said fetch address of said second cache line if said fullsignal indicates the instruction buffer is full when the instructioncache provides said second cache line for storage in the instructionbuffer.
 9. The branch control apparatus of claim 8, wherein said backupregister provides said fetch address of said second cache line on theaddress bus if said full signal indicates the instruction buffer is nolonger full.
 10. The branch control apparatus of claim 1, wherein saidaddress register provides said target address to the instruction cacheafter said second cache line is stored in the instruction buffer if afetch address of said second cache line misses in the instruction cache.11. A pipelined microprocessor, comprising: an instruction cache,coupled to an address bus configured to receive a first fetch addressfor selecting a first cache line; a branch target address cache (BTAC),coupled to said address bus, for storing and directly providing a wrapindicator for indicating whether a branch instruction wraps beyond saidfirst cache line; an address register, coupled to said BTAC, for storinga target address of said branch instruction, said target addressprovided by said BTAC; and a multiplexer, coupled to receive said targetaddress from said address register and coupled to receive a second fetchaddress that specifies a second cache line containing a portion of saidbranch instruction wrapping beyond said first cache line, wherein ifsaid wrap indicator is true, said multiplexer selects said second fetchaddress for provision onto said address bus and subsequently selectssaid target address from said address register for provision onto saidaddress bus, after selecting said second fetch address for provisiononto said address bus, wherein said BTAC provides said target address inresponse to said first fetch address received on said address bus,wherein said BTAC provides said target address in parallel with saidinstruction cache providing said first cache line.
 12. The pipelinedmicroprocessor of claim 11, wherein said multiplexer selects said secondfetch address after selecting said first fetch address.
 13. Thepipelined microprocessor of claim 11, wherein said second fetch addressis a next sequential fetch address to said first fetch address.
 14. Thepipelined microprocessor of claim 11, wherein said multiplexer selectssaid target address for selecting a third cache line in said instructioncache, said third cache line containing a target instruction of saidbranch instruction.
 15. The pipelined microprocessor of claim 14,wherein said multiplexer selects said first fetch address, said secondfetch address and said target address for causing said first, second andthird cache lines to be stored in order in an instruction buffer.
 16. Abranch control apparatus in a microprocessor, comprising: a branchtarget address cache (BTAC), for caching indications of whetherpreviously executed branch instructions wrap across two cache lines andfor directly providing said indications; a register, coupled to saidBTAC, for receiving from said BTAC a target address of one of saidpreviously executed instructions stored therein; and control logic,coupled to said BTAC, for receiving one of said indications associatedwith said one of said previously executed branch instructions; whereinif said one of said indications indicates said one of said previouslyexecuted branch instructions wraps across two cache lines, said controllogic causes the microprocessor to branch to said target address, aftercausing said two cache lines containing said one of said previouslyexecuted branch instructions to be fetched, wherein said control logicis configured to cause a first of said two cache lines containing aportion of said one of said previously executed branch instructions tobe fetched from an instruction cache, wherein said control logic isconfigured to receive a miss indicator from said instruction cache,wherein said control logic is configured to cause a second of said twocache lines containing a portion of said one of said previously executedbranch instructions to be fetched from a memory other than saidinstruction cache if said miss indicator indicates said second of saidtwo cache lines is missing in said instruction cache.
 17. The branchcontrol apparatus of claim 16, wherein said control logic is configuredto cause a third cache line to be fetched from said instruction cacheafter said second of said two cache lines, said third cache linecontaining a target instruction of said one of said previously executedbranch instructions.
 18. The branch control apparatus of claim 17,wherein said third cache line is fetched from said instruction cacheusing said target address stored in said register.
 19. A microprocessorbranch control apparatus, comprising: an incrementer, coupled to aninstruction cache address bus, for providing a first fetch address onsaid address bus, said first fetch address selecting a first cache linecontaining a first portion of a branch instruction; a branch targetaddress cache (BTAC), coupled to said address bus, for providing atarget address of said branch instruction in response to said firstfetch address, and for caching and directly providing an indication ofwhether said branch instruction wraps beyond said first cache line; anaddress register, coupled to said BTAC, for storing said target addressif said BTAC indicates said branch instruction wraps beyond said firstcache line; and a flag register, for storing an indication that saidaddress register stores said target address of said branch instructionthat wraps beyond said first cache line; wherein said incrementerprovides a second fetch address on said address bus, said second fetchaddress selecting a second cache line containing a second portion ofsaid branch instruction; wherein said address register provides saidtarget address on said address bus, said target address selecting athird cache line containing a target instruction of said branchinstruction; wherein if said flag register indicates that said addressregister stores said target address of said branch instruction thatwraps beyond said first cache line, and if said second fetch addressmisses in an instruction cache coupled to said address bus, saidincrementer provides a third fetch address sequential to said secondfetch address on said address bus, and said address register does notprovide said target address on said address bus.
 20. A method forperforming branches in a microprocessor with an instruction cache, themethod comprising: storing in a branch target address cache anindication of whether a previously executed branch instruction wrapsbeyond a first cache line containing at least a portion of said branchinstruction, and said branch target address cache directly providing theindication; applying a first fetch address to the instruction cache forselecting said first cache line containing at least a portion of abranch instruction, after said storing said indication; providing atarget address of said branch instruction and said indication inresponse to said first fetch address; determining whether said branchinstruction wraps beyond said first cache line based on said indication;storing said target address in a register if said branch instructionwraps beyond said first cache line; applying a second fetch address tothe instruction cache, if said branch instruction wraps beyond saidfirst cache line, for selecting a second cache line containing aremainder of said branch instruction; providing said target address fromsaid register to the instruction cache for selecting a third cache linecontaining a target instruction of said branch instruction; and cachingsaid target address of said branch instruction after a previousexecution of said branch instruction and prior to said providing saidtarget address of said branch instruction in response to said firstfetch address.
 21. The method of claim 20, further comprising: cachingan indication of whether said branch instruction wraps beyond said firstcache line after execution of said branch instruction and prior to saidapplying said first fetch address.
 22. The method of claim 21, whereinsaid determining whether said branch instruction wraps beyond said firstcache line comprises examining said indication.
 23. The method of claim20, further comprising: determining whether said second fetch addressmisses in said instruction cache.
 24. The method of claim 23, furthercomprising: obtaining said second cache line from a memory other thansaid instruction cache if said second fetch address misses in saidinstruction cache.
 25. The method of claim 24, wherein said providingsaid target address from said register to the instruction cache isperformed after said obtaining said second cache line from said memoryother than said instruction cache.
 26. The method of claim 20, furthercomprising: storing said first, second and third cache lines in aninstruction buffer.
 27. The method of claim 26, wherein said storingsaid first, second and third cache lines in said instruction buffercomprises storing said first, second and third cache lines in saidinstruction buffer in order.